in the constellation Ursa Major
6.5 million to 20 million times the mass of the Sun
Diameter roughly 30 to 100 times the size of the Sun
Astronomers have developed several techniques for discovering the supermassive black holes in the centers of galaxies and for measuring their masses. Comparing several methods in a single galaxy allows them to verify that each technique works, providing more confidence in the measurements for galaxies in which only one or two techniques can be used.
One galaxy for which several techniques have been used successfully is Arp 151, an odd-looking galaxy that is not far from the Big Dipper in Earth's sky.
In the early 2000s, astronomers measured the mass of the black hole in the galaxy's center by measuring the motions of stars near the black hole. The speed at which the stars orbit the galaxy's center indicates a heavy, compact object such as a black hole or a dense star cluster. No star cluster or other bright object was detected, however, leaving a supermassive black hole of about 20 million times the Sun's mass as the most likely candidate.
Another team used a second technique, in which the astronomers determined the mass of the galaxy's central "bulge" of stars. Observations of many galaxies has shown a relationship between the mass of the bulge and the mass of the central black hole. Using this relationship, the astronomers also calculated a mass of roughly 20 million times the mass of the Sun.
Later, another team used a third technique, called reverberation mapping. The technique compares the spectra of gas in the large, bright accretion disk around the black hole with thinner clouds of gas at greater distances from the black hole. The accretion disk is so bright that its radiation causes the more distant gas clouds to glow.
This technique takes advantage of the fact that accretion disks don't always shine at the same brightness. A disk can flare brightly as new material falls into the disk or as magnetic fields cause some of the disk's gas to clump together. Measuring how long it takes the surrounding clouds to brighten as they're illuminated by these flares reveals their distance from the black hole. And measuring the width of the lines in the spectra from these clouds reveals how fast they are moving. From these two measurements -- distance and velocity -- astronomers then calculate the mass of the central black hole.
The reverberation technique gives a smaller mass for Arp 151's central black hole, of about 6.5 million to 7 million times the mass of the Sun, or less than twice as massive as the central black hole in the Milky Way.
The estimates using the three techniques vary by a factor of about three. When you account for the possible errors in each technique, however, the gap is much smaller and can be accounted for by small errors in observations or differences in the models used to simulate the system. When those are taken into consideration, the techniques are in fairly close agreement, providing confidence that all three are offering an accurate picture of the supermassive black hole at the center of Arp 151.
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This document was last modified: May 11, 2012.